Authentication information (massage authentication code) termed as message authentication code (MAC) is commonly used to authenticate packets transferred between terminal apparatuses over a network. When a packet is transferred from a source apparatus to a destination apparatus via multiple relaying apparatuses located along the packet transfer route, the source apparatus transmits the packet with attachment of multiple MACs (message authentication codes) that correspond respectively to the multiple relaying apparatuses along the route. The attachment of the MACs to the packet enables the relaying apparatuses to verify the packet in turn.
The use of a MAC-authentication technique makes it possible to prevent any evil-minded attacker from gaining unauthorized access to the terminal apparatuses via the multiple relaying apparatuses located along the communication route. In addition, the use of the MAC-authentication technique also makes it possible to detect a fraudulent tampering of a packet.